1. Field of the Invention
The present invention relates to an ink jet recording apparatus equipped with a sheet conveyance mechanism having an upstream side conveyance unit arranged on the conveyance upstream side of a recording head and a downstream side conveyance unit arranged on the conveyance downstream side thereof.
2. Description of the Related Art
As a form of a recording apparatus such as a printer, a copying machine, or a facsimile apparatus, an ink jet recording apparatus is used which performs recording by discharging ink onto a sheet from a discharge port of a recording head based on image information. Further, in a recording apparatus, there is widely employed a construction equipped with a sheet conveyance mechanism having an upstream side conveyance unit arranged on the conveyance upstream side of a recording head and a downstream side conveyance unit arranged on the conveyance downstream side thereof. Here, a state in which a sheet is pinched (or nipped) between both the upstream side conveyance unit and the downstream side conveyance unit is referred to as a first conveyance state, and a state in which the sheet is pinched solely by the downstream side conveyance unit is referred to as a second conveyance state. When the recording proceeds from the first conveyance state to reach a state in which recording can be performed on a sheet trailing edge portion, the sheet trailing edge portion leaves the position where it has been pinched by the upstream side conveyance unit. At this time, the sheet is switched to the second conveyance state, in which it is held solely by the downstream side conveyance unit. Then, since the downstream side conveyance unit is configured to convey the sheet at a higher speed than the upstream side conveyance unit, the sheet is conveyed at a higher speed than in the first conveyance state. Thus, when the conveyance state is switched, excessive conveyance occurs, which may lead to deterioration in sheet image quality such as white streak or color misregistration.
In this connection, Japanese Patent Application Laid-Open No. 2008-200893, for example, discusses a control method in which correction is effected with respect to the second conveyance state. Japanese Patent Application Laid-Open No. 2008-200893 discusses a control method according to which a test pattern is recorded when conveyance in both the first and second conveyance states has been completed (in other words, when the sheet trailing edge has left the upstream side conveyance unit), and when conveyance is performed solely in the second conveyance state. In this control method, according to the test pattern recording result, a correction amount, in which, for example, individual differences in the recording apparatus and the sheet are taken into account, is calculated, thus effecting the correction of the conveyance amount.
However, in the above related art, when the offset amount of the rollers of the upstream side conveyance unit and of the downstream side conveyance unit is large, the effect of the correction is reduced, making the recording result rather unstable in some cases. In particular, regarding a discharge roller as a drive side roller used for the downstream side conveyance unit, the offset amount is larger as compared with that of a conveyance roller as a drive side roller used for the upstream side conveyance unit. More specifically, the discharge roller is generally configured such that a plurality of rubber rollers is engaged with a metal shaft, so that it involves a rather large offset amount. Further, elastic deformation (tremor) of the metal shaft also causes an increase in the offset amount. Further, as the metal shaft of the discharge roller, generally the shaft having lower rigidity as compared with the conveyance roller is used. Generally speaking, a roller pair of the upstream side conveyance unit is formed by the conveyance roller and a pinch roller which is a driven rotary member, and a roller pair of the downstream side conveyance unit is formed by the discharge roller and a spur which is a driven rotary member.
The discharge roller is configured to perform conveyance at a higher speed as compared with the conveyance roller. Thus, in the first conveyance state, in which conveyance is performed by both the conveyance roller and the discharge roller, the discharge roller having lower rigidity, is attracted to the conveyance roller side. And, when switching to the second conveyance state, the deflection of the discharge roller that has been attracted is released. The behavior at this time varies according to the deformation (tremor) state of the metal shaft. More specifically, when the sheet is conveyed solely by the downstream side conveyance unit, if the rotating position where the discharge roller is in contact with the sheet is different from the rotating position at the time of the test pattern recording for conveyance amount correction, the conveyance correction amount may be inadequate.